When Doom targets an enemy, they will die exactly 3 rounds after the turn this special was used (e.g if Doom is used in round 3, the doomed unit will die in round 6). A doomed enemy will still die even if the targeting Lord of Ashes is killed before or on the turn that Doom is resolved. The only way to escape its effect is to end combat before the resolving round, either by retreat or killing all opposing units.